1. Field of the Invention
This invention relates to an image coding system, more specifically to an interframe coding system, wherein an image signal is coded on a frame-to-frame difference basis.
2. Discussion of the Related Art
It is a common practice to remove redundant components in an image signal for a highly efficient coding. Especially with a dynamic image signal, an interframe coding is one of the preferable and typical arts. The interframe coding is a coding technique which codes a difference between a newly input uncoded image signal and a previously coded image signal.
FIG. 40 is a block diagram of a conventional interframe coding apparatus disclosed in Japanese Unexamined Patent Publication No.208382/1988. The interframe coding apparatus according to the figure has a frame memory 1, a motion-vector detector 2, a subtractor 3, an encoder 4, a local decoder 5, an adder 6, a filter 7 and a filter controller 8. The frame memory 1 stores an image signal of the previous frame.
Operation of the conventional interframe coding system with a local decoding loop is now described with reference to the FIG. 40.
The input image signal 12 is compared with the image signal of the previous frame 11 stored in the frame memory 1 by a block-matching technique in the motion vector detector 2. The motion vector detector 2 detects the quantity and direction of motion of the input image signal 12 and outputs a motion vector signal 13. The frame memory 1 outputs a motion compensation predictive signal 14 based upon the motion vector signal 13. The subtractor 3 subtracts the motion compensation predictive signal 14 from the input image signal 12 to output a predictive error signal 15 or a difference signal.
The predictive error signal 15 is coded by quantization at the encoder 4, and a coded error signal 16 is output. The coded error signal 16 is decoded in the local decoder 5, and a local decoded error signal 17 is output. The local decoded error signal 17 is added to the motion compensation predictive signal 14 in the adder 6, and a local decoded signal 18 is output. The local decoded signal 18 is filtered by the filter 7 to eliminate higher frequency components in the signal. The filter smooths the local decoded signal and outputs a smoothed local decoded signal 19. Filtering is controlled by a control signal 20 output by the filter controller 8. The filter control signal 20 controls the filter based upon the motion vector signal 13.
The coded error signal 16 and the motion vector signal 13 are transmitted via a transmission line to an external decoding system.
Image coding is generally processed by the unit or block of 16.times.16 or 8.times.8 pixels of an image signal.
With reference to other conventional coding techniques, the filter 7 can be placed after the frame memory 1 in the local decoding loop instead of after the adder 6 as shown in FIG. 40. Further, filtering can be accomplished by an intra-block filter which processes pixels within a block, and by an inter-block filter which processes pixels involving the pixels in the neighboring blocks. Furthermore, a motion detection can be achieved with a smaller unit of a pixel than a full pixel. This is designed to detect the optimal block of pixels in the previous frame which match a block of pixels in the input image signal 12. These conventional techniques can contribute to eliminate higher frequency components or redundancy in the input image signal based upon the quantity of motion. Thus, they are effective for removing noise in the signal. Consequently, coding efficiency can be improved greatly with these conventional arts.
An intraframe coding with a local decoding loop is processed in the following manner according to the conventional coding with reference to FIG. 1. The input image signal 12 is directly coded by quantization in the encoder 4, where the coded error signal 16 is output. The coded error signal 16 is decoded in the local decoder 5, where the local decoded error signal 17 is output. The local decoded error signal 17 is stored in the frame memory 1. The coded error signal 16 is transmitted via a transmission line to an external decoding system.
The conventional interframe coding generally controls filtering based upon the motion vector, whereby an image signal can be filtered based upon the quantity of motion. In other words, a block of pixels representing motion of an image signal, or a motion block, is filtered with a low-pass filter (LPF) thereby eliminating the original definition to eliminate noise in the signal. On the other hand, a block of pixels representing no motion or a very small amount of motion, is not filtered.
FIG. 41 illustrates filtering different characteristics of the motion compensation predictive signal 14 using the LPF in relation with luminance intensity (I) versus frequency (f). In the figure, 14a designates a characteristic of the image signal from the frame memory 1 without LPF filtering. 14b designates a characteristic of the image signal from the frame memory 1 after LPF filtering.
In other words, a picture with no or little motion is unfiltered to keep its original definition as characteristic 14a shows. An image signal in motion is filtered with the LPF reducing its original definition to eliminate higher frequency components in the signal as characteristic 14b shows. Thus, LPF filtering eliminates higher frequency components, designated by the shaded portion in FIG. 41, in the image signal based upon the quantity of motion of the image signal.
FIGS. 42(a), 42(b) and 42(c) respectfully illustrate LPF filtering characteristics of the input image signal 12, the motion compensation predictive signal 14a, and the predictive error signal 15. When the motion compensation predictive signal 14a can reproduce the input image signal 12 successfully, as shown in the figure, according to the excellent motion detection performance, the difference or the predictive error signal 15 between the two signals is small. Consequently, this can contribute to a higher coding efficiency because the amount of coding information is reduced.
Accordingly, when a motion block of the motion compensation predictive signal has a high quality of reproduction of the input image signal with the excellent motion detection performance and the difference between the two signals is very small, the block need not to be filtered. However, this does not apply to some cases: an image signal is filtered irrespective of the reproduction quality of the motion compensation predictive signal.
When the motion compensation predictive signal 14 is filtered with the LPF, the signal loses higher frequency components as shown in FIG. 41. As a result, the difference between the input image signal and the filtered motion compensation predictive signal is the higher frequency components. In other words, a large amount of coding is needed thereby reducing the coding efficiency and resulting in a low definition.
FIGS. 43(a), 43(b) and 43(c) respectfully illustrate the characteristics of two input signals and one output signal at the subtractor 3. FIG. 43(a) shows the characteristic of the input image signal 12, which corresponds to that of FIG. 42 (a). FIG. 43(b) shows the characteristic of the filtered motion compensation predictive signal 14b in comparison to that of the unfiltered motion compensation predictive signal 14a. This shows that the higher frequency components (noise) in the signal with motion is eliminated by the LPF filtering. FIG. 43(c) shows the characteristic of the predictive error signal 15 which is the difference between the two input signals 12 and 14. The difference still contains the higher frequency components of the motion compensation predictive signal 14. This requires the encoder to have to encode bigger amounts of information leading to a lower coding efficiency.
Thus, the problem is stemmed from the unnecessary filtering as FIGS. 42(a), 42(b), 42(c), 43(a), 43(b) and 43(c) illustrate. The motion compensation predictive signal 14 is not to be filtered when the signal has a high quality of reproduction due to a high detection performance of motion as shown in FIGS. 42(a), 42(b) and 42(c). When the motion compensation predictive signal 14 is filtered unnecessarily under that condition as shown in FIGS. 43(a), 43(b) and 43(c), the image is damaged involving poor coding efficiency due to a higher amount of coding information or involving poor definition.
The conventional interframe coding system carries another problem of quantization. Coding performance, according to the conventional system, is based upon a limited quantization which is designed optimally for a certain pattern of predictive error signal 15. In other words, the limited quantization can not deal effectively with coding signals of various patterns. When the encoder quantizes the signal with the limited quantization, the predictive error signal 15 is characterized with a poor coding efficiency and results in producing a poor coded error signal.
The present invention is de voted to solve the problems. An objective of this invention is to provide an interframe coding system with a high coding efficiency by eliminating higher frequency components remaining in a coded signal.
Another objective of this invention is to provide a coding controller which allows the encoder to code efficiently with coding signals of various patterns. This can lead to the overall coding efficiency of the interframe coding system.